"Send To" only works if the desired destination is listed.
Ok...I'm a computer idiot, too. How do I "just copy" them?
Just open up the Widows Explorer to view the contents of your drives. You should be able to see both your Seagate drive and also the directories on your computer's hard drive where your files to be backed-up are located. It is easiest if you open up the Explorer twice, once in one window to see the Seagate drive, and a second time to see and find your files. Then, to copy from one to the other, just select the directories/files you are wanting to copy (i.e. highlight them by clicking your mouse on them, holding "shift" or Ctrl" key to select multiple files) and then drag them over to the Seagate drive where you drop them by releasing your mouse button.
Not so easy in Vista or 7. All documents are kept in "Libraries", and are not accessible directly from the C drive. But the Libraries are listed on the left when opening up the "Computer" icon, and the files are accessible from there. And there are many ways to copy files. I'm not a big fan of click and drag. But after you highlight the files you want to copy, you can either hit "Control and C"(copy), go to the folder or drive you want the copies to be in and hit "Control and V"(paste). Or, instead of the "Control C or V" thing, you can just right click on the highlighted files and select either Copy or Paste. With click and drag, it's not easiest to have the two Explorer windows open, it's required.
And while we're in the "neighborhood", totally moving files is just as easy. After you highlight the desired files, hit "Control and X"(cut), or right click and select "cut", and then "Control and V(paste) or right click and select "paste" as before. And the difference is that "Copy" does just that, makes a copy that you now have in two places, whereas "Cut" moves your one and only file from one place to another.
That'll be 5 cents, please.